1. Field of the Invention
The present invention relates to an overcurrent detection circuit which, when an overcurrent flows to an output transistor that outputs current to a load in a regulator that converts a supply voltage into a predetermined DC voltage, for example, detects the overcurrent and provides protection, and also relates to a regulator including such an overcurrent detection circuit.
2. Description of the Related Art
A regulator such as a switching regulator or series regulator has an output transistor provided between a supply voltage that is input and a terminal that is connected to the load and outputs a predetermined DC voltage. The regulator maintains a predetermined DC voltage by controlling the output transistor. Further, in order to prevent the output transistor from being damaged by an overcurrent in the event of an anomaly such as when there is a short-circuit of the load, a circuit to detect the overcurrent and implement protection is provided (See, for example, Japanese Patent Application Laid-open No. H8-331757).
FIG. 4 shows a conventional overcurrent detection circuit and a regulator that includes the conventional overcurrent detection circuit. The regulator 101 is constituted by an output transistor 11 of a P-type MOS transistor, a smoothing circuit 12 that smoothes the output of the output transistor 11, an output terminal OUT that outputs a smoothed predetermined DC voltage, a control circuit 14 that controls the output transistor 11 by inputting the voltage of the output terminal OUT as feedback, and an overcurrent detection circuit 110a that detects the overcurrent of the output transistor 11 and implements protection. A load 13 is connected to the output terminal OUT.
The overcurrent detection circuit 110a is constituted by a monitor transistor 121 of a P-type MOS transistor, the source of which is connected to a supply voltage Vcc and the gate of which is connected to the gate of the output transistor 11, an output current detection resistor 122, one end of which is connected to the drain of the monitor transistor 121 and the other end of which is grounded, an overcurrent detection output transistor 123 of an N-type MOS transistor, the gate of which is connected to an interconnect between the drain of the monitor transistor 121 and the output current detection resistor 122, the drain of which is connected to the control circuit 14, and the source of which is grounded. Here, the size of the monitor transistor 121 is set at 1/N that of the output transistor 11. The current flowing to the monitor transistor 121 is only used in the overcurrent detection. Therefore, N is a natural number with a large value to minimize the current value so that the power loss is not increased.
Now, when an output current Io flows to the output transistor 11, a current having a value of substantially Io/N flows to the monitor transistor 121 and a corresponding voltage is produced in the output current detection resistor 122 with ground potential serving as a reference. Further, when the output current Io become an overcurrent and the voltage exceeds a threshold value (Vth) of the overcurrent detection output transistor 123, the overcurrent detection output transistor 123 outputs a low level. As a result, the control circuit 14 turns OFF the output transistor 11 since the output current of the output transistor 11 exceeds the overcurrent detection level (since it is determined that an overcurrent is flowing). Here, the overcurrent detection level can be adjusted by the size of the monitor transistor 121 and the resistance value of the output current detection resistor 122.
Thereafter, another conventional overcurrent detection circuit and a regulator including the overcurrent detection circuit are shown in FIG. 5. The regulator 102 has substantially the same constitution as the regulator 101 apart from the overcurrent detection circuit 110b. The overcurrent detection circuit 110b also includes the monitor transistor 121 and output current detection resistor 122 as per the overcurrent detection circuit 110a above. The overcurrent detection circuit 110b includes an overcurrent detection output comparator 125 instead of the overcurrent detection output transistor 123. The overcurrent detection output comparator 125 has an inversion input terminal connected to the interconnect between the monitor transistor 121 and output current detection resistor 122, a non-inversion input terminal connected to an overcurrent detection reference voltage 124, and an output terminal connected to the control circuit 14.
When the output current Io flows to the output transistor 11, a current of substantially Io/N flows to the monitor transistor 121 and the corresponding voltage is produced in the output current detection resistor 122 with the ground potential serving as the reference. When the output current Io becomes an overcurrent and the voltage exceeds the overcurrent detection reference voltage 124, the overcurrent detection output comparator 125 outputs a low level. As a result, the control circuit 14 turns OFF the output transistor 11 since the output current of the output transistor 11 exceeds the overcurrent detection level. Here, the overcurrent detection level can be adjusted by the size of the monitor transistor 121, the resistance value of the output current detection resistor 122, and the value of the overcurrent detection reference voltage 124.
Thus, the overcurrent detection circuits 110a and 110b are able to detect the overcurrent flowing to the output transistor 11. The present inventor, who conducted research into further improving the accuracy of the overcurrent detection level of the overcurrent detection circuit, focused attention on the voltage produced in the output current detection resistor 122 in the overcurrent detection circuits 110a and 110b, that is, the drain voltage of the monitor transistor 121 is produced with ground potential as a reference, whereas the drain voltage of the output transistor 11 is produced not with ground potential as a reference. That is, the voltage of the drain of the monitor transistor 121 accordingly differs from the voltage of the drain of the output transistor 11 and, as a result, it is assumed that the current of the monitor transistor 121 is different from the value Io/N.
Therefore, although consideration was paid to adjusting the overcurrent detection level by calculating the difference beforehand and changing the resistance value of the output current detection resistor 122, because the magnitude of the displacement also varies when the input supply voltage Vcc fluctuates, such a countermeasure is ineffective.
Furthermore, the present inventor also focused on the effect on the overcurrent detection level resulting from the fact that the monitor transistor 121 and output transistor 11 have different temperature characteristics from the output current detection resistor 122 when the temperature fluctuates. Further, because the characteristic of the overcurrent detection output transistor 123 in the overcurrent detection circuit 110a also fluctuates, it is assumed that the overcurrent detection level further fluctuates.